1. Field of the Invention
The present invention relates to an amplifier circuit for amplifying internal signals or input/output signals of electronic equipment and devices or, in particular, to an amplifier circuit constituting an output stage of an operational amplifier used for signal processing in an analog electronic circuit.
2. Description of the Related Art
In conventional class-A operational amplifier circuits, the no-load output potential is set to a point intermediate between a supplied voltage using a current of a constant current source and a current supplied from a current-conversion transistor for converting the current of the output voltage of a differential amplifier circuit. The current of the current-conversion transistor and the current of the constant current source determine the driving capability of the output of the particular amplifier circuit. The larger the magnitude of these currents, therefore, the more the impedance can be reduced that can drive the signal amplitude as a load. Under no load or under a voltage load requiring no output current, however, the currents of a current-conversion transistor flow directly from the high potential side to the ground, thereby increasing the current consumed by the circuit. These currents must be reduced in order to reduce the power consumption of the amplifier circuit. The problem, however, is that a reduced current leads to a reduced drive capability.
Further, an attempt to increase the dynamic range (amplitude range) of the output current is accompanied by the requirement of considerably changing the output voltage of the differential amplifier circuit supplying a gate voltage to the current conversion transistor. For the output voltage amplitude of the differential amplifier circuit to be increased, on the other hand, the offset voltage between two input signal terminals increases unavoidably. As a result, an offset also occurs in the signal applied through the circuit, thereby posing the problem of crossover distortion.